The ultrasound pulse-echo technique is widely used in medical imaging. This imaging method currently uses an array of transducer elements to transmit a focused beam into the body, and each element then becomes a receiver to collect the echoes. The received echoes from each element are dynamically focused to form an image. Focusing on transmission and on reception is done assuming that the velocity inside the body is uniform, and is usually assumed to be 1540 ms.sup.-1. Unfortunately, the velocity inside the body is not constant; it varies from 1470 ms.sup.-1 in fat to greater than 1600 ms.sup.-1 in some other tissues, such as collagen, and 1540 ms.sup.-1 is therefore an approximation. This variation will result in increased side lobes and degraded lateral resolution. Aberration phenomena and their extent in tissue have been evaluated in many works. The degradation is tolerable if the frequency is not very high and the aperture is not very large. Recently, however, higher frequencies and larger apertures have been used to improve lateral resolution of ultrasound images. But the resolution improvement can not be achieved beyond a certain limit, because both larger aperture and higher frequency make the system more sensitive to propagation velocity variations in the body. It is therefore one of the major difficulties in improving lateral resolution of ultrasound imaging systems.
Methods have been developed to correct aberrations in such areas as atmospheric effects in astronomical imaging, antenna-position errors in radar and microwave imaging, weathered-layer-effect in seismic imaging, and subcutaneous body layers effects in medical ultrasound imaging. Some of these methods use the wavefront from a special target such as a dominant point target or a spectacular reflecting plane, to measure the phase-aberration profile. Some methods measure the phase-aberration profile using signals from arbitrary target distributions, and signal redundancy is the basic principle behind these methods. Even though the signal redundancy principle has been successfully used in astronomical imaging to correct aberrations introduced by atmospheric turbulence, its application in medical ultrasound imaging systems has achieved only moderate success because the near-field effect has not been analysed in sufficient detail.
Amplitude aberrations in ultrasound medical imaging have also been previously reported, particularly for imaging complex tissue structures such as the female breast. Amplitude aberration will influence the quality of images, even though it is perhaps not as important as phase. In some cases, amplitude aberration correction is needed in addition to phase aberration correction to obtain a good image. The present invention seeks to also incorporate an amplitude aberration correction with the phase aberration correction algorithm.
Review of Aberration Correction Methods
Astronomical imaging systems are passive and incoherent. Phase and amplitude aberrations caused by the atmosphere make it difficult to achieve diffraction-limited resolution on the ground. A widely used aberration-correction method is the direct wavefront measurement method. This is used when there is a dominant bright star either present or artificially created [R. K. Tyson, "Principle of adaptive optics." Academic Press, ch. 5, 1991]. A plane wavefront at the aperture should be observed since the dominant point target is in the far field of the imagine aperture. Any departure from a plane wavefront is caused by phase aberration. Many methods can be used to measure a wavefront, one of which is the interferometer wavefront sensing method. After measuring the wavefront, the next step is to separate the phase-aberration profile across the aperture from the non-aberrated wavefront. This is not a simple task because the target angle is generally unknown. Fortunately, for a target in the far field, such separation is not necessary since the error of the assumed target angle causes a shift of the image position only and has no effect on focusing, the phases need only to be adjusted so that the wavefront from the dominant target is a plane wave.
Methods using signals received from arbitrary target distributions have also been developed in astronomical imaging. These include maximum-sharpness [R. A. Muller and A. Buffington, "Real-time correction of atmospherically degraded telescope images through image sharpening," J. Opt. Soc. Am., vol. 64, no. 9, pp. 1200-1209, September 1974] [A. Buffington, F. S. Crawford, R. A. Muller, A. J. Schwemin, and R. G. Smits, "Correction of atmospheric distortion with an image-sharpening telescope," J. Opt Soc. Am., vol. 67, no. 3, pp. 298-303, March 1977] and redundant-spacing interferometer [R. C. Jennison, "A phase sensitive interferometer technique for the measurement of the Fourier transforms of spatial brightness distributions of small angular extent," Mon. Not. R. Astron. Soc., vol. 118, no. 3, pp. 276-284, 1958 [M: Ishiguro, "Phase error correction in multi-element radio interferometer by data processing," Astron. Astrophys. Suppl. Ser., vol. 15, pp. 431-443, 1974] methods. Hamaker et al. [J. P. Hamaker, J. D. O'Sullivan, and J. E. Noordam, "Image sharpness, Fourier optics, an redundant-spacing interferometry," J. Opt. Soc. Am., vol. 67, no. 9, pp. 1122-1123, August 1977] pointed out that these methods are all based on the same fundamental principle: signal redundancy. When the target distribution is complex, there is no prior knowledge about the wavefront shape without phase aberrations. Therefore, the aberration profile can not be separated directly from the unaberrated wavefront in the measured wavefront. In this case, fortunately, the signal redundancy principle makes the separation unnecessary. The redundant-spacing interferometer method does not measure the wavefront but the phase difference between redundant signals, and then directly derives the phase-aberration profile across the array. The result also contains an arbitrary steering angle which has no effect on focusing. The maximum-sharpness method uses a trial-and-error method to adjust each antenna's phase. When an indicator is maximised, the system is in focus. This method is also based on the signal redundancy principle. When the redundant signals are in phase, they sum coherently and the indicators will be maximised. The signal redundancy principle for targets in the far field will be reviewed in detail hereinafter.
Phase-aberration correction methods have also been developed for active (pulse-echo), coherent and near-field imaging systems such as radar, microwave, ultrasonic and seismic systems. When there is a dominant point target in the near-field, the first step is again to measure the arrival wavefront from the target. Nearest neighbour cross-correlation and indicator (maximum sharpness) methods have been use. The next step is to separate the aberration profile from the unaberrated wavefront, which should be spherical. Without knowing the position of the dominant point target, the phase-aberration profile can only be obtained by estimating the target locations. The error in the estimated aberration profile because of the wrongly assumed target position will cause de-focusing in the near-field case. The image at the dominant point target will still be well focused (at the wrong position) if this inaccurate aberration profile is used to do the correction, since the two errors cancel each other at that position. But, when moving away from that point, the correction will become increasingly inaccurate. This correction is therefore only valid in a region around the dominant target. The size of the region depends on the distance from the target to the aperture, the size of the aperture, and the accuracy of the estimated target position. It can be much smaller than the isoplanatic patch defined as the region where the phase-aberration value is a constant, if the focusing quality is too poor before phase-aberration correction to estimate the dominant point target position with adequate accuracy. Therefore, aberration correction in the near-field may have problems even when a dominant point target is available. In some situations, such as forming an image around the dominant target only [B. D. Steinberg, "Microwave imaging of aircraft," Proc. IEEE, vol. 76. no. 12, pp. 1578-1592. December 1988], estimating the dominant target position accurately is not so important. On the other hand, it is unusual to have a dominant point target in every isoplanatic patch, or even in the entire image, in medical ultrasonic imaging.
Techniques have also been developed which use signals from randomly distributed scatterers that generate speckle in an image to measure the wavefront. In the correlation method [M. O'Donnell and S. W. Flax, "Phase-aberration correction using signals from point reflectors and diffused scatterers: measurement," IEEE Trans. Ultrason., Ferroelect., Freq. Cont., vol. 35, no. 6, pp. 768-774, November 1988], a focused beam is transmitted. If the transmitted beam is perfectly focused on a point, the reflections from the small focal spot mimic a dominant point target. The reflected wavefront can be measured, and the phase-aberration profile can be derived if the position of the focus point is known. But, since the array has only a finite aperture size, and the phase aberration is not yet known, the transmitted beam will not be perfectly focused and its pattern is not known. This pattern will result in signals received at different elements to be different (The Van Cittert-Zernike theorem has been used to predict the differences [H. R. Mallart and M. Fink, "The Van Cittert-Zernike theorem in pulse echo measurements," J. Acoust. Soc. Am., vol. 90, no. 50, pp. 2718-2727, November 1991] [D. L. Liu and R. C. Waag, "About the application of the Van Cittert-Zernike theorem in ultrasonic imaging," IEEE Trans. Ultrason., Ferroelect., Freq. Cont, vol. 42, no. 4, pp. 590-601, July 1995].) This difference will introduce an additional phase distribution across the array on top of phase distributions produced by a point target and phase aberrations. These three components mixed together make it very difficult to measure the phase-aberration profile accurately. From another point of view, the phase-aberration values associated with each element are mixed together in the transmitted beam and it is difficult to measure them separately using the received signals.
The indicator method has also been used in a speckle-generating region in near-field imaging systems. The indicator(maximum sharpness) method was developed for optical astronomy [R. A. Muller and A. Buffington, "Real-time correction of atmospherically degraded telescope images through image sharpening," J. Opt. Soc. Am., vol. 64, no. 9, pp. 1200-1209, September 1974] [A. Buffington, F. S. Crawford, R. A. Muller, A. J. Schwemin, and R. G. Smits. "Correction of atmospheric distortion with an image-sharpening telescope." J. Opt. Soc. Am., vol. 67. no. 3, pp. 298-303, March 1977].
When redundant signals are in phase with one another, the indicator is maximized. It is the intensity-sensitive recorder that generates the necessary cross-correlation process between signals coming from different locations on the lens aperture to produce redundant signals in a passive imaging system. Phases of redundant signals are difficult to compare directly for a lens system. A trial-and-error method has to be used with a deformable lens to focus the image by maximizing an indicator, this is time consuming and it may not converge to the right position. On the other hand, in a very large baseline, radio astronomy imaging system, phases of redundant signals can be compared directly. It is more reliable than the indicator method to measure phase-aberration values related to each element. In ultrasonic imaging, radio-frequency signals can be collected and their phases can be compared directly. Therefore, direct-phase difference measurement between redundant signals should be used. The inventor herein shows that in the near field, common midpoint signals, which are redundant for active far-field imaging systems, are no longer identical. Therefore, the indicator method, as well as the signal redundancy method, using the whole aperture, generally can not be used. The inventor has herein proposed a near-field signal redundancy algorithm is proposed and tested.
In seismic imaging, a phase-aberration correction (surface-consistent residual static correction) method using signals coming from a specular reflecting plane has been developed to correct the phase aberration caused by weathered layers near the ground surface. The specular reflecting plane is a special kind of target. It is similar to a dominant point target in that every element receives a dominant echo from it. The difference is that the position of the reflecting point is different for different transmitter or receiver positions. The non-aberrated arrival wavefront from a specular reflecting plane depends on the angle of the plane and the propagation velocity between the plane and the array. Common receiver, common transmitters and common midpoint signals can be used for the measurement. Common midpoint signals are generally preferred because of several advantages such as small residual normal-move-out components and insensitivity to the angle of the reflecting plane [J. A. Hileman P. Embree, and J. C. Pflueger. "Automated static correction." Geophys. Prosp., vol. 16, pp. 326-358, 1968]. It should be noted that even though common midpoint signals are redundant signals for far-field targets in an active imaging system (discussed later), they are not redundant when there is a specular reflecting plane in the near field, because the position of the reflecting point is different for different transmitter or receiver positions. The seismic method is not a signal redundancy method. A signal redundancy method measures the phase-aberration profile directly. The seismic method measures the arrival wavefront from a specular reflecting plane first, then separates the aberrated and the non-aberrated wavefronts. A least-mean-square error fitting method, which uses the complete data set, has also been developed to measure the phase-aberration profile and form an optimally stacked section image in seismic imaging, which measures the phase-aberration profiles, the velocity above the plane, and the angle of the plane together [M. T. Taner, F. Koehler, and K. A. Alhilali. "Estimation and correction of near-surface time anomalies," Geophys., vol. 39, no. 4, pp. 442-463, August 1974]. This method could be used to estimate the dominant point target position as well as the phase-aberration profiles in the dominant point target case, when tang into account that the position of the reflecting point is unchanged when the transmitter or the receiver position changes.
A least-mean-square error-fitting method has also been developed in ultrasonic imaging [M. Hirama and. T. Sato, "Imaging through an inhomogeneous layer by least-mean-square error fitting," J. Acoust. Soc. Am. vol. 75, no 4, April 1984] to form an image of targets on a plane parallel to the transducer array surface through an inhomogeneous layer. The method uses the complete signal set to build an over-determined equation group which has sufficient equations to estimate the spatial frequency components of the target plane and the aberration profiles across the array. The technique requires the area of the target to be small; when the system is in the signal zone, only an approximated image can be obtained. The method will not apply to targets that extend in range. A least-mean-square error-fitting method using the signal redundancy principle to measure the phase-aberration profile directly, has also developed [D. Rachlin, "Direct estimation of aberration delays in pulse-echo image Systems," J. Acoust. Soc. Am. vol. 88, no. 1, July 1990]. Basically, it is a far-field signal redundancy technique and does not adequately address the near-field effect
In medical ultrasound imaging, dominant points, specular reflecting planes, and large area of uniformly distributed speckle-generating target distributions are unlikely to be found in every isoplanatic patch. The signal redundancy method, which relies very little on target distributions, seems attractive. But, before it can be used in medical ultrasound imaging systems, the near-field effect has to be considered. First, however, the signal redundancy principle for targets in the far-field will be reviewed.
Signal Redundancy Principle in the Far Field
When a sensor array with many small aperture sensors is used to synthesize a larger aperture, it is possible to collect identical signals using different sensors from arbitrary target distributions. These signals are called redundant signals. In a passive array-imaging system, the cross-correlation functions between any two signals received from elements separated by the same distance (off-set) are identical if targets are in the far field and spatially incoherent, as shown in FIG. 1. This is because cross-correlation functions between signals from different sources vanish as a result of incoherence, and the cross-correlation functions between signals from the same source are identical because the phase shift is the same. Phase-aberrations will change the relative positions of these identical cross-correlation functions, and corrections can be derived from the relative time shift between them.
A similar result occurs for an active array-imaging system, such as an ultrasound synthetic aperture imaging system using an array. The received signals are identical for arbitrary jet distributions if the middle point position of the transmitter and the receiver is the same, and targets are in the far field as shown in FIG. 2. This is because the distance from a to b is equal to the distance from C to d at all angles.
The above signal redundancy principle is valid for continuous waves as well as for wide-band signals. It is valid generally for arbitrary target distributions provided that targets are in the far field and the medium is homogenous. The signal redundancy property of common offset signals in passive imaging systems results in the effective aperture being based on cross-correlations between transmission and reception apertures. The signal redundancy property of common midpoint signals in active imaging systems results in the effective aperture being based on convolutions between transmission and reception apertures.
When the propagation velocity in a medium is inhomogeneous, phase aberrations are introduced. If the phase aberration caused by velocity inhomogeneity can be modelled as a phase screen on the aperture, these redundant signals will still have the same shape, but will be shifted according to the phase aberration experienced by each signal. The phase-aberration profile across the array can be measured from the relative time shift between these redundant signals.